Ten vehicles were involved in two separate crashes within a few minutes of each other on the Brighton bypass.
The first happened west of the A27-A23 junction at 8.20am when eight vehicles collided.
Minutes later, two vehicles crashed on the east side of the junction a short distance away near the Hollingbury turn off, Brighton.
Police said there were no reports of serious injuries.
Traffic backed-up for several miles until the road was cleared.
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